Opamp upgrade - compensation cap ?

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redrabbit

redrabbit

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Joined 2004
#1
Several times I've come across schematics using TL084 , 82 opamps, with a compensation cap across the feedback resistor, in mixer applications and other apps.
The TLOxx datasheets say they are internally compensated.

I'm wanting to build these circuits, using my more modern opamps, say OPA4134, 2134.

What do I do with that comp cap ?
Example:
http://www.musicfromouterspace.com/analogsynth/stereomixer.html

More specific questions:
(my guesses in parenthesis)

How does one determine if a cap is needed? (high gain? )

Determining the cap's value ? (math? or a phase meter?)

thanks,
=RR=
 
#2
Hi, Good question, the cap you refer to is included to roll of the response outside the audio bandwidth, and also if optimised correctly will help reduce any ringing or overshoot on square wave testing. The cap posseses what is called reactance and put simply you can think of this as a resistor whose value depends on the applied frequency. It is a bit more complex than that but it explains the point. Reactance. = 1/2*pi* F*C . That is to say 1 Divided by 2* PI* Frequency * Value of cap in Farads. So in your example it is a component that decreases in "resistance" as the frequency rises and so the gain around the OpAmp is reduced.
Regards Karl
Edit, Sorry didn't answer your question, if you use other Op-Amps the value of the cap is unchanged. You cannot really determine the exact value without fully analysing the complete circuit untill it is constructed since the value of stray capacitance will have to be taken into account. It's not a problem in applications like this provided the value is not so big it affects the response in the audio band. Do you have a scope and signal generator ? Some wide bandwidth OpAmps may require the cap for stability, TL072 OPA62604 etc dont.
 
#4
In this case/circuit those caps are there to compensate stray/layout capacitance at the -IN pins of the opamps, which could cause instabilty. 22pF seems right for the mixer amps with all the distrubuted stuff hooked to them, for the output amps it's bit high though and causes HF roll-off, which here is tolerable. Better to be on safe side.

When you build this, I'd make sure all those 22k's after the panpots are close to the opamps, although the site says otherwise.

A very good discussion about that can be found here (Section 4):
http://apex.cirrus.com/en/pubs/appNote/Apex_AN19U_C.pdf

- Klaus
 
#6
It's very instructive to build a simple amp (one opamp 2 resistors) and test it with different values of cap and different values of feedback resistor. You can get a feel for and see on the 'scope how squarewave response is affected. Aim to get the response of any circuits flat to at least 30KHz, but overshoot and ringing on squarewave test not good.
Regards Karl
 
#7
the accurate term is "integrating" cap. compensation takes place in the voltage amp stage of an op amp. compensation affects the actual speed of the op amp, integration affects the frequency response of the feedback loop. since TL06x, 7x, and 8x op amps are internally compensated
 
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